It is almost self-evident that test results will be unreliable and misleading if those undergoing assessments do not make a full effort on testing. Nevertheless, objective tests of effort have not typically been used with young adults to determine whether test results are valid or not. Because of the potential economic and/or recreational benefits of obtaining the diagnosis of attention deficit hyperactivity disorder (ADHD) or a learning disability (LD), concerns have been raised regarding the ease with which unimpaired young adults can feign either of these disorders to gain access to test accommodations, stimulant medication, or disability benefits. Much evidence has been presented recently regarding the need for symptom validity tests (SVTs) in assessment of college-aged students seeking diagnoses of LD and/or ADHD. Four cases are presented here in which intelligence and other test scores of young adults greatly underestimated their actual abilities, owing to poor effort that sometimes went undetected. Selected effort tests for use with young adults are discussed. Objective testing of effort is recommended to avoid misinterpreting invalid test data, which is why the use of effort tests is now standard practice in forensic neuropsychology. (Contains 2 tables.)